1. Field of the Invention.
The present invention relates to a DC motor controller utilizing full-wave rectified alternating current (AC) for the motor drive power, and more particularly, to DC motor controller that will control the speed and other related parameters such as torque of a direct current (DC) motor.
2. Description of the Related Art.
Many devices and methods to control speed of a DC motor have been designed in the past. None of them, however, include a circuit design that starts the conduction at AC zero (or near zero) and stops conduction at some higher AC voltage to allow speed control from zero (rotor stop) to some maximum rotor speed rotation.
One commonly accepted method to control speed of a DC motor is to vary the direct current input voltage to the motor. This requires a relatively expensive and complicated DC power supply, since some form of DC voltage/current regulation is commonly expected.
Speed can also be controlled from the AC line using thyristors to turn the AC signal on and off. This method is well known but has some problems with smooth start torque (from zero, or rotor stop) due to the thyristor requirements of turning on (conduction start) at some relative high voltage in the AC cycle and turning off (conduction stop) at AC zero voltage. Turn on time is typically 5 microseconds and turn off time is about 25 microseconds. To avoid detecting false turn on/turn off times, a rather complicated snubber circuit is required. This is variously referred to as time interval modulation, phase control, pulse width control, etc., depending on the scheme of controlling conduction time and/or applied voltage. Speed control is typically in the 50:1 range.
With the present invention, the DC motor accepts a unipolar current/voltage of constant DC voltage or a rectified AC signal. i.e. as long as the signal is unipolar and the current into the motor only conducts in one direction, the motor operates effectively. When the voltage is increased, the speed increases (the motor rotates faster). If the voltage reverses, both the current and the direction of rotation reverses. A DC motor is near a "constant current" device. As voltage (or ON time) is increased, rotational speed increases directly as the voltage (or ON time) but the motor current remains nearly constant (if load is constant, which is the typical motor application) over the voltage (or ON time) control range. This is due to the reverse EMF of the rotor coil and inertia of the rotor/load characteristics.